430 stainless corrosion and finish compatibility

430 stainless corrosion performance is a narrow promise: it resists staining and light attack in dry indoor service, and it stops being a corrosion material the moment chloride and standing moisture arrive. The grade contains 16–18% chromium and no nickel and no molybdenum, which puts its pitting resistance equivalent near 17, against 18.6 for 304 and 25.4 for 316L.

Most 430 is bought for the surface, not the metallurgy. It polishes to a bright, slightly warmer tone than an austenitic grade, it is magnetic, and it is the cheapest stainless sheet on the coil. The finish is therefore the product, and the question a drawing has to answer is whether that finish survives the environment it is being sold into.

Two things follow. Bare 430 belongs indoors, and coated 430 belongs indoors unless the coating is genuinely intact for the whole service life. Neither is a defect in the grade; both are a specification that has to be made deliberately.

Where does bare 430 stainless corrosion hold up?

It performs well in heated and dry indoor air, adequately in an unheated dry interior, and poorly from the moment condensation, chloride or a crevice enters the picture.

The reason is the passive film. Chromium oxide forms on 430 as it does on any stainless, but without molybdenum the film is slower to repair after a chloride attack, and without nickel the alloy has less tolerance for the reducing conditions inside a pit or a crevice. A 430 surface that is scratched in a dry room repassivates in air within minutes. The same scratch in a coastal wash-down becomes a pit in weeks.

The ISO 9223 classification is the practical way to scope this. The categories run from C1 (heated, dry, low sulphur) through C2 (unheated dry), C3 (urban or industrial with moderate pollution), C4 (coastal or heavily industrial), C5 (splash and salt fog) to CX (offshore and tropical marine). 430 is comfortable in C1 and C2, marginal in C3, and wrong in C4 and above.

The visible failure is tea staining: a brown discolouration that follows the water run-off pattern on a panel. It is not structural, but it is the complaint that arrives first, and it cannot be polished out on a powder-coated part.

The last column is not a sales limit; it is where the grade stops working. A 430 panel in a C5 plant is a warranty claim that has already been designed.

Why 430 pits where 304 does not

The difference is molybdenum, not nickel. Both grades carry 16–18% chromium, and both form a passive film; molybdenum is what makes that film resistant to chloride breakdown at a pit site.

That is why 304 survives a coastal interior that destroys 430, and why 316L survives a wash-down that stains 304. The PREN scale tracks it: 430 at 17, 304 at 18.6, 316L at 25.4, 444 at about 24. A 430 part with a 1% molybdenum addition, sold as 434, moves to about 20 and buys a little margin, but it does not become a marine grade.

Crevices are the other multiplier. A gasket line, a hem, a riveted lap or a self-clinching fastener creates a gap where oxygen is consumed faster than it can be replaced, and the chloride concentration inside the gap rises. 430 in a crevice will pit at chloride levels that leave an open surface untouched. If the design has a lap joint, the grade decision and the joint design have to be made together.

Powder coating 430: the substrate that takes a conversion coating

430 is the easier stainless to coat, and the reason is iron. A conventional zinc phosphate bath needs iron at the surface to build the conversion crystals, and a 430 sheet provides it after a light etch. The same bath on 316L needs a stronger acid stage or a zirconium-based conversion, because the austenitic surface is more passive.

The route we run is the standard one: alkaline degrease, rinse, activation, conversion, rinse, demineralised final rinse, dry-off on the seven-tank line, then powder in the 18 ft × 14 ft booth and cure in the oven. The 16 ft × 16 ft blast chamber precedes it where the part needs a profile.

Two preparation details matter more on 430 than on CRCA:

• Blast profile. Ra 2–3.5 µm with angular alumina gives the film a mechanical key. Shot or glass bead peens the surface and closes the profile, which shows up as adhesion loss at the first cross-hatch.
• Edge condition. A laser-cut 430 edge is hard, sharp and oxide-covered. Deburr it, and expect the film to pull thin at the corner; a 40 µm minimum on the edge is the callout that keeps the corrosion front away from the steel.

Film stress is slightly higher on 430 than on 316L because the substrate expands less. Polyester expands at roughly 40–60 × 10⁻⁶ per kelvin against 10.4 × 10⁻⁶ for 430. Across a 175 K cure and cool cycle that is about 0.69% strain against 0.6% on 316L, so a thick film is under more tension on the ferritic substrate. The practical answer is the same: 60–80 µm, not 120.

Adhesion is checked by cross-hatch to ASTM D3359 Method B, and the first article should also get a bend test around a 6 mm mandrel if the panel will be handled or flexed in service.

Wet paint on 430, and the primer that does the work

Wet paint is the right route for a large 430 panel, for a part with exposed edges, or for a substrate that cannot take a 200 °C cure. A two-pack epoxy primer at 25–40 µm bonds to the conversion coating and provides the barrier; the topcoat provides colour and UV resistance.

The failure mode to design out is primer delay. An epoxy primer has a recoat window, and a panel that sits for a week before topcoat needs the primer keyed before the next coat. On a thin 430 panel, the handling between coats also matters more than on steel, because a 0.8 mm ferritic sheet dents at a lower impact energy than the same gauge in CRCA.

Wet paint gives better edge coverage than powder, which is why it is the better choice for a part with many laser-cut openings and a C3 exposure.

Galvanising 430, and the galvanic couple nobody draws

430 cannot be galvanised. The zinc bath at about 450 °C does not wet a chromium oxide surface and forms no zinc-iron intermetallic layer, so the coating runs off and the part comes out bare and heat-marked. A drawing that says "430, hot-dip galvanised" is a contradiction.

Where a zinc finish is genuinely needed, the material is CRCA. A 1.5 mm CRCA part galvanised after fabrication gives a zinc-iron coating that protects cut edges by sacrificial action, which no stainless finish can do.

The galvanic couple matters in the other direction too. Stainless is cathodic to zinc, so a 430 panel bolted to a galvanised frame makes the frame the anode. If the stainless area is large and the galvanised area small, the zinc corrodes quickly at the joint, especially where water sits. Keep the stainless part small relative to the galvanised one, or isolate the interface, and do not assume that two corrosion-resistant materials are compatible just because each works alone.

Aluminium contact is the same story with a smaller potential difference. A 430 panel riveted to an aluminium extrusion in a wet environment will corrode the aluminium at the joint, and the white corrosion product stains the stainless.

Bare finish options and what brushing does to 430

The finish designations come from ASTM A480. 2B is the standard cold-rolled bright finish, BA is bright annealed and smoother, and No. 4 is a brushed grain produced with a 120–150 grit abrasive.

430 brushes to a slightly different tone than 304, and the difference is visible when the two are used on the same facade. The grain also reveals inclusions and roping on a ferritic surface, so a No. 4 finish on 430 needs a mill with tight surface control, not a belt sander in a fabrication shop.

Passivation follows ASTM A967. On bare 430 the treatment removes free iron and restores the chromium oxide film, and it matters most after laser cutting, where the edge carries oxide and a chromium-depleted zone. Citric acid is adequate for light contamination; a pickling paste is needed on a weld-affected zone, and the residue has to be rinsed thoroughly or it becomes the next corrosion site.

Polishing to a mirror finish is possible but rarely economic on 430. The grade does not take as high a lustre as 304, and a mirror 430 panel in a humid room shows every fingerprint and every tea stain.

Masking and the fit change from 60 µm of powder

Coating is a dimension on 430 as it is on any substrate. At 70 µm per face a bolted lap gains 0.14 mm, a 6.4 mm hole for an M6 fastener closes to about 6.26 mm, and a 0.2 mm clearance in a locating slot disappears.

Mask threads, earth studs, sliding faces and any zone that will be welded after coating. On a 430 enclosure the earth stud matters more than usual, because the powder is the only thing between the panel and a floating bond. Keep a 10 mm bare ring around the stud and check continuity with a low-resistance ohmmeter.

How Ganesh finishes 430 parts

430 panels run through the same line as the rest of the sheet work: 12 kW fibre laser with the 40 ft × 16 ft bed for cutting, the 200 T 5-axis brake with the 4,450 mm bed for forming, and the 16 ft × 16 ft blast chamber, seven-tank pretreatment, 18 ft × 14 ft powder booth and curing oven for finishing. DFT is checked with a gauge and adhesion with cross-hatch, both in-house, and the plant holds ISO 9001:2015 and ISO 14001:2015.

The coating capability page lists the booth and oven envelope, and the custom parts range covers 430 parts from 0.8 mm to the top of the thickness range we cut. Where the finish has to survive an outdoor enclosure, the guide to powder failures on outdoor enclosures sets out the pretreatment and edge causes.

What to put on the 430 drawing

1. Grade and standard: 430 to ASTM A240 or IS 6911, with the heat number on the certificate.
2. Exposure class: the ISO 9223 category, or the chloride level and wetness in plain words.
3. Finish route: bare passivated, powder coated, or primed and painted, with one route named.
4. Surface finish before coating: 2B, BA or No. 4, and the Ra if the face is visible.
5. Film thickness with the edge minimum, and the cure schedule by metal temperature.
6. Passivation standard where the part stays bare, and the test that proves it.
7. Masking schedule, including the earth stud and every thread.

Send the drawing with the exposure class written on it to request a quotation. The finish route for a 430 part is decided by where it will hang, not by what colour the buyer wants, and the two decisions are cheaper to separate at quotation than at the first warranty claim.

When 430 and a coating is the wrong combination

It is wrong outdoors, in a wash-down, on a part that will be welded after coating, and on any part where the coating is expected to survive an impact that will breach it. A breached coating on 430 has no sacrificial layer beneath it, so the steel at the damage site is exposed and the surrounding film holds the moisture against it.

That is the case for 316L, or for a different material class entirely. The 316L finish compatibility guide covers the austenitic route, and the powder coating on laser-cut edges covers the edge detail that decides whether either grade keeps its finish. The materials and grades hub collects the grade-selection set, including the case against 430.